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Figure 2 (A) Shown are simultaneous recordings from surface lead II, Rove catheter records from the left septum, His bundle recording, and distal coronary sinus (CS)_1,2 and proximal CS_3,4 recordings. The earliest activation site in the left septum was 30 ms before the onset of the surface P-wave. (B) Application of radiofrequency energy to the earliest left septal site during tachycardia results in acceleration before return of sinus rhythm.